1. Field of the Invention
The invention relates to a method of electrochemically machining an electrically conductive work piece in an electrolyte by applying bipolar electric pulses between the work piece and an electrically conductive electrode, one or more voltage pulses of unipolar machining polarity being alternated with voltage pulses of opposite polarity, while a gap between the work piece and the electrode is maintained, which gap is filled by the electrolyte.
The invention further relates to an arrangement for electrochemically machining of an electrically conductive work piece in an electrolyte by applying bipolar electric pulses between the work piece and an electrode, one or more voltage pulses of unipolar machining polarity being alternated with voltage pulses of an opposite polarity, while a gap between the work piece and the electrode is maintained, which gap is filled by the electrolyte.
2. Description of the Related Art
Electrochemical machining is a process in which an electrically conducting work piece is dissolved at the location of an electrode while electrolyte and electric current is supplied. For this purpose, the electrode is brought in the proximity of the work piece and, while electrolyte is fed into the gap between the work piece and the electrolyte a powerful current is passed through the work piece and the electrode via the electrolyte, the work piece being positive with respect to the electrode. The current is applied in the form of machining pulses having a given amplitude and duration. In the intervals between the machining pulses the electrolyte is renewed. Under the working conditions the work piece is being dissolved, thus increasing the value of the gap between the work piece and the electrode. To compensate for this, the electrode and the work piece are moved towards one another with a given feed rate, as a result of which the electrode forms a cavity or eventually a hole in the surface of the work piece, the shape of the cavity or hole having the shape corresponding to the shape of the electrode. This process can be used, for example, for making intricate cavities or holes in or for shaping hard metals or alloys. The copying precision with which the shape of the cavity or the hole in the work piece corresponds to the shape of the electrode is important for the quality of the result.
A method to perform electrochemical machining where bipolar voltage pulses are utilized is known from U.S. Pat. No. 5,833,835. A pulse component of opposite polarity to that of the machining pulses is used to remove deposits on the front surface of the electrode. The amplitude of the pulses of the opposite polarity is limited by the condition of a wear of the surface of the electrode. This condition is checked by performing a test based on a value of the polarization voltage between the work piece and the electrode after a termination of machining pulses.
The disadvantage of the known method is the fact that the process of the removal of cathode depositions is not controlled, as there is no information available in the system upon the extent of the cathode depositions. Therefore, it is possible that the surface of the cathode is not completely cleaned, leading to deviations in the effective geometrical shape of the cathode. This results in an inferior accuracy of the electrochemical machining.
It is an object of the invention to provide a method to improve the accuracy and the efficiency of the electrochemical machining, due to an improved process of determination of cathode depositions followed by a controlled removal of the depositions from the front surface of the electrode.
The method according to the invention is characterized in the steps of:
performing a measurement of a first value of an operational parameter that depends on the cleanness of the front surface of the electrode, the first value corresponding to a clean front surface of the electrode;
performing a measurement of a second value of the operational parameter in an interval between the unipolar machining voltage pulses after at least one unipolar machining voltage pulse is applied;
performing a computing of a deviation between the first value and the second value of the operational parameter; and
application of at least one voltage pulse of the opposite polarity only after the computed deviation is non-zero.
According to the technical feature of the invention a measurement of the first value of the operational parameter corresponding to a condition of a clean electrode surface is performed and is further used as a reference value in order to derive the extent of the cathode depositions. It is understood that after application of machining pulses depositions occur on the front surface of the electrode. This phenomenon is pronounced especially under conditions of difficult evacuation of the products of chemical reactions taking place in the gap. The depositions thus formed comprise mainly hydrates and oxides of chemical elements present in the work piece. The mechanism of a deposition formation and its removal under bipolar electrochemical machining is as follows. It is understood that the metals, for example Fe, Ni, Al, Ti, Cr are ionized in water solutions of salts under electrochemical machining of different types of metals. These ionized metals are transported by the electrolyte flow in the vicinity of the cathode, where they form oxides, hydroxides and salts, for example Fe(OH)3, Cr(OH)3, Ni(OH)2, Al(OH)3, FeOH(NO3)2, Fe(OH)2NO3. These compositions further lead to a formation of positively charged colloids, like [mFe(OH)3nFe3+(n-x)OHxe2x88x92]2+. The underlying chemical reactions form a basic knowledge for a person skilled in the art. When these colloids reach the surface of the cathode, they are deposited there in a form of cathode depositions. The main physical processes for the formation of the cathode depositions are electrophoretic transport of positively charged particles and their further adsorption at the cathode surface. In the bipolar mode, the anode processes occur at the cathode surface, when the voltage pulses of the opposite polarity are applied to the gap. This process is characterized by an intense oxygen formation, according to a reaction:
2H2O-4exe2x86x92O2+4H+
The oxygen formation leads to the removal of the depositions from the cathode surface first by means of mechanical rupture of the layer of the depositions. Secondly, an acid is formed in a vicinity of the cathode, with a pH-value in the order of 1-2. Several chemical reactions occur at the surface of the cathode leading to a further removal of the depositions under the influence of the acid thus formed. Thus, the front surface of the cathode is cleaned, as a result of a mechanical influence of the oxygen formation accompanied by a chemical dissolution of the depositions in the formed acid layer.
The chemical and phase composition of cathode depositions is determined by the material of the work piece (anode) and can differ from the material of the electrode (cathode). Thus, properties of the cathode are varied, if the depositions of a different element composition occur on the surface of the cathode. By selecting an appropriate property of the cathode as the operational parameter, the generation of the depositions can be detected. This provides the possibility to obtain the information about the extent of the formation of the depositions on the front surface of the electrode. Thus, one obtains a quantitative information about the extent of the cathode depositions by performing on-line measurements of the operational parameter and by comparing the measured value with a reference value corresponding to a clean surface of the cathode. This information is used in the method according to the invention in order to perform a removal of the cathode depositions in an accurate and controlled way by means of an application of voltage pulses of the opposite polarity if the deviation between the first value and the second value of the operational parameter has occurred. The amplitude, the pulse upslope and the duration of the pulses of the opposite polarity are selected in such a way that, for a given value of the gap, an intense acid formation in the vicinity of the front surface of the cathode takes place, the thickness of the acid region being sufficient to dissolve the depositions and to maintain the dissolved depositions in the electrolyte. An example of such operational conditions is achieved for the pulse upslope not greater than 2 xcexcs, resulting current density value in the gap of at least 1000 A/cm2, and pulse duration in a range of 5 to 20 xcexcs.
An embodiment of the method according to the invention is characterized in that if the computed deviation between the first value and the second value of the operational parameter is greater than an a-priori defined third value, each successive unipolar machining voltage pulse is followed by a voltage pulse of the opposite polarity for a number of repetitions. From the point of view of the efficiency of the electrochemical process, it is preferable to define a threshold upon which the action for the removal of the cathode depositions must be undertaken. This threshold is quantified by the third value of the operational parameter. In case it is detected that this threshold is surpassed, the system switches to a different mode, where the unipolar machining pulses are alternated with the pulses of the opposite polarity for a number of repetitions. This number of repetitions can be pre-set according to an empirically established value dependent upon the third value of the operational parameter. It is also possible that the application of the voltage pulses of the opposite polarity is stopped when it is detected that the value of the operational parameter reached a predetermined cut-off value.
A further embodiment of the method according to the invention is characterized in that if the computed deviation remains greater than the third value after the application of the number of repetitions, the pulse duration of the voltage pulses of the inverse polarity is increased by a predetermined increment. This technical measure is based on the insight that the efficiency of the removal of the cathode depositions is an integral effect, depending on both the operational conditions of the electrochemical process, like the value of the gap and the electrolyte flow on one hand and on the amplitude and the duration of the applied voltage pulse of the opposite polarity on the other hand.
A still further embodiment of the method according to the invention is characterized in that a value of the electrode potential of the front surface of the electrode is selected as the operational parameter. This technical measure is based on the insight that the chemical and phase composition of cathode depositions is determined by the material of the work piece (anode) and can differ from the material of the electrode (cathode). Thus, the electrode potential is varied, if the depositions of a different element composition occur on the surface of the cathode. This provides the possibility to obtain the information about the formation of the depositions on the front surface of the electrode. By measuring the cathode potential prior to the electrochemical machining or within the first number of machining pulses, the value of the cathode potential corresponding to the surface without the depositions is determined. As the depositions are formed on the surface of the cathode due to the processes described above, the absolute value of the cathode potential is changed. By performing periodic measurements of the cathode potential, it is possible to derive the amount of the generated depositions. The measurements are preferably performed in an interval between the machining pulses. It is further possible to set a predetermined allowable deviation between the initial value of the cathode potential and the actual measured value. This determines the allowable toleration of the geometrical shape of the front surface of the cathode. In case the computed deviation is greater than the pre-set allowable deviation, pulses of the opposite polarity are applied. The details will be further explained with reference to the figures.
A still further embodiment of the method according to the invention is characterized in that in a region, corresponding to an interval between the unipolar machining voltage pulses, an area under a curve of the electrode potential is derived, said area being selected as the operational parameter. This choice of the operating parameter is preferable for electromechanical machining conditions with high electrolyte flow or for small electrolyte paths, or for larger values of the gap. In these conditions, the operating conditions are not favorable for the cathode depositions to be generated to a great extent, resulting in a minor variations in the value of the cathode potential. It is more sensitive to detect the formation of the cathode depositions by analyzing the curve of the electrode potential. Further details will be given with reference to the figures.
A still further embodiment of the method according to the invention is characterized in that the absolute value of the first harmonics of the Fourier transformation of the cathode potential pulse is selected as the operational parameter. This technical measure is based on an insight that the absolute value of the first harmonics of the Fourier transformation is a direct measure of a height of the cathode depositions of the surface of the electrode. Further on, it is understood that, for very short intervals between the pulses of the machining polarity, the coefficients of the Fourier transformation are more sensitive to the cathode depositions than the absolute value of the cathode potential. Additionally, it is possible to monitor the actual value of the first harmonics of the Fourier transformation and to use it as a system control parameter for avoiding an electrode wear. This technical measure is base on an insight that the value of the first harmonics has a positive sign in case the cathode depositions are generated. In case the surface of the electrode is being dissolved as a result of an application of the pulses of the inverse polarity, the sign of the first harmonics of the Fourier transformation changes from xe2x80x98plusxe2x80x99 to xe2x80x98minusxe2x80x99. Therefore, by monitoring the absolute value of the first harmonics and/or its sign, it is possible to perform the cleaning of the electrode surface from the cathode depositions without inducing electrode wear.
A still further embodiment of the method according to the invention is characterized in that for short intervals between the unipolar machining voltage pulses in a region, corresponding to an interval between the unipolar machining voltage pulses, a slope of a curve of the electrode potential is derived, the slope being selected as the operational parameter. It has been found that the value of the cathode potential is not stabilized between the machining pulses for a high-frequency electromechanical machining. Therefore, it is preferable to use a quantitative characteristic of the course of the curve of the electrode potential. This quantitative characteristic is the slope of the curve, given by:             k      φ        =                  (                              φ            2                    -                      φ            3                          )                    (                              φ            1                    -                      φ            2                          )              ,
where xcfx861 is the value of the electrode potential at a time moment i after the machining voltage pulse has been switched off.
By choosing the equidistant sample times for measuring of the corresponding values of the electrode potential in order to determine the slope of the curve, the process control can be further simplified.
The method according to the invention improves the copying accuracy of the electrochemical machining due to the fact that the geometric shape of the electrode is preserved essentially unchanged. The process efficiency can thus be increased by about 20% due to a decrease of the resistance of the gap, obtained surface quality of the work piece is improved by one class.
These and other aspects of the invention will be further discussed with reference to the figures.